1. Field of the Invention
The invention relates to an arrangement for the heating of the ink in the laminated write head of an ink-jet printer, which is constructed based on the layering technique.
2. Brief Description of the Background of the Invention Including Prior Art
Individual ink droplets are conventionally ejected from the nozzles of a write head, under the control exercised by an electronic control system where the write head is part of an ink-jet printer for the production of characters on a recording substrate. Characters and/or graphic patterns are generated on a recording substrate as grids within a character matrix by tuning the ejection of individual droplets and the relative motion between the recording substrate and the print head. The operational safety and the quality of the recording depend to a large extent on the uniformity of the droplet ejection, i.e. the individual droplets ejected based on a control pulse have to exhibit a defined size and have to leave the nozzle of the write head in each case with the same speed. The boundary conditions for formation of a uniform droplet ejection are multifaceted. For example, the ink drop formation or the ink-jet formation, respectively, the ink droplet mass and the flight motion velocity of the ink drops in such printers depend to a large extent on the viscosity of the ink. Since the viscosity of the ink depends on temperature, the ink has to be sufficiently well controlled with respect to temperature by way of a heating device, in order to assure, on the one hand, that an ink ejection process is at all possible in case of different temperatures and, on the other hand, that this ink ejection process is performed as defined and as stable as possible. For this purpose, it is already known to maintain the temperature of the ink in an ink write head at a constant value. For a write head, where individual ink channels are furnished and where the individual ink channels end at the ejection nozzles of a nozzle plate, it is known from the German Patent Application Laid-Open DE-OS 2,659,398, to furnish a heating element in the nozzle plate. Furthermore, it is known for such write heads to dispose an induction coil in the area of the nozzle plate and to heat the nozzle plate by eddy currents and remagnetization losses, of. German Patent Application Laid-Open DE-OS 3,500,820.
Ejection of individual ink droplets occurs in high-resolution ink-jet printers according to the so-call bubble-jet principle, and the write head is formed and constructed in such printers according to the thin-layer technology, by generating an ink-vapor bubble in the respective ink channel in the region of individually controllable electrothermal energy converters disposed in the ink channels. The generated vapor bubble ejects a certain predetermined ink volume as a droplet out of the ink channel.
The temperature dependence of the viscosity of the ink is a very important factor for write heads of this kind. Consequently, it is also known for write heads of the recited kind to improve the ejection conditions by a preheating of the ink. This can be performed by employing additional, external heating elements acting on the ink from outside a channel, such as described for example in the German Patent Applications Laid Open DE-OS 2,943,164 or DE-OS 3,545,689. Frequently, resistors with a positive temperature coefficient are employed as heating elements. Thus, the temperature of the ink in the write head can be brought to and maintained at a certain elevated predetermined temperature value in connection with a control circuit and with a temperature sensor element, where the temperature sensor element can frequently be a negative temperature coefficient resistor. However, relatively long heating times result in particular in connection with write heads with electrothermal converters. The reason for this is that steps and means for cooling have to be provided for write heads with electrothermal converters because of the accompanying heating of the ink occurring during the ongoing printing operation. For this purpose, the print head is usually disposed at a cooling surface, for example, on an aluminum plate. If after longer intervals between printing operations, or in case of the switching-on of the ink-jet printer, the ink has to be heated, then it is always necessary that the cooling surface be heated up at the same time. Based on this, there result relative long heat-up times. In addition, the construction and production-technological expenditures associated therewith are not negligible, since in each case additional individual elements have to be kept ready, mounted, and electrically connected.
It is already known from the German Patent Application Laid-Open DE-OS 2,943,164 to dispose a heating coil in the interior of the ink volume space. In addition to the construction expenditures, there result however, also problems based on chemical processes occurring between the coil material and the ink liquid.
In addition, it is conceivable to dispose an ink heater and temperature sensor at the bubble-jet write head in an additional plane of the thin film substrate. However, such a structure is associated with several production-technological and economical disadvantages affecting and related to the reliability, yield, and processing times of such print heads, since additional process steps are required for this purpose, such as deposition, lacquer deposition, illumination, development, etching, photoresistant layer degradation, covering, etc. In addition, a certain failure probability exists based on electric short circuits between the two large-faced conductor arrangements for the heating element, the sensor and for the bubble-jet structure, furnished by an aluminum conductor structure, where the conductor arrangements are separated only by a thin oxide, typically of a thickness of about 2 micrometers.